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Aims:Daily exposure to 100 p.p.m. chlorine dioxide of single species and binary bio®lms of dairy-associated Bacillus cereus DL5 and Pseudomonas¯uorescens M2, attached to stainless steel surfaces in a laboratory¯ow system, was studied. Methods and Results: Surfaces were sampled daily before and after sanitizer treatment and cells and spores dislodged and enumerated by standard methods. Duplicate surfaces were prepared for confocal scanning laser microscopy (CSLM) and scanning electron microscopy. Higher counts of Ps.¯uorescens M2 were obtained in single species bio®lms, microcolonies stained green (viable) in CSLM images and were closely packed on attachment surfaces. By contrast, higher counts of B. cereus DL5 were obtained in binary bio®lms, microcolonies stained green in CSLM images, but were more spread out. Lower spore counts were obtained for B. cereus DL5 in binary bio®lms. The survival of Ps.¯uorescens M2 cells after exposure to chlorine dioxide was apparently enhanced by the presence of B. cereus DL5 in binary bio®lms. By contrast, B. cereus DL5 showed increased susceptibility to sanitizer treatment in the presence of Ps.¯uorescens M2. Conclusions: Co-cultured bacteria in bio®lms in¯uence each other with respect to attachment capabilities and sanitizer resistance/susceptibility. Signi®cance and Impact of the Study: Binary bio®lms endemic in food-processing industries can survive sanitation regimes and may represent reservoirs of product contamination leading to subsequent spoilage and/or food safety risks.

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